I've only ever come across the word in Aldous Huxley's book The Doors of Perception, where it's spelt 'mescalin'. I've never seen it spelt with an 'e' at the end (although I defer to Tom44's expert knowledge in the area and accept his view that the more common spelling is 'mescaline'). Perhaps to those of us without expert knowledge, 'mescalin' is more common. It is to me, at least.